(1) Field of the Invention
The present invention relates to a transmission system of image data from a plurality of sources wherein the image data from the plurality of sources are respectively thinned to limit a total transmission rate before transmitting the image data.
The present invention can be applied to a television conference system having a plurality of television cameras in each conference room, or security service system for monitoring a plurality of scenes taken by a plurality of television cameras, by displaying the respective scenes in turn on one television monitor
(2) Description of the Related Art
In a television conference, image data obtained in two or more conference rooms are transmitted therebetween. In recent television conference systems, image data are obtained by a plurality of television cameras in each conference room, are multiplexed in a television conference terminal apparatus provided in each conference room, and are then transmitted through a transmission line having a constant transmission rate, to the other conference rooms. Although, generally, signals from the plurality of conference rooms, respectively indicating image data, are coded to reduce the transmission rates of the respective signals, it is further required to thin the respective image data frames by discarding a part of successive image frames in each of the image data signals from the plurality of sources.
Conventionally, the above thinning (discarding) is carried out at constant rates for the respectively image data signals. Namely, constant transmission rates are allocated to the respective image data signals.
However, a maximum allowable rate of the above thinning (discarding) for maintaining a minimum required quality of a moving image (i.e., a quality of a moving image after thinned, coded, transmitted, and then decoded on a receiver side) regarding smoothness of movement of an object in the image--varies with time, depending upon a degree of movement of an object in the image from each source (television camera). Therefore, according to the above conventional technique wherein the constant thinning (discarding) rates are allocated to the respective image data from the plurality of television cameras, the quality of the moving image may fall below the above minimum quality when the degree of the movement becomes high. Conventionally, to prevent such a problem, the above thinning (discarding) rates allocated to the respective image data signals must be limited to low rates. However, since a total transmission rate of each transmission line connecting the plurality of conference rooms is predetermined, the decrease in the above thinning rates forces the coder to decrease the output bit rate thereof. Namely, in this case, the quality of the image is reduced by the decrease in the output bit rate of the coder.